When a thick cloud of air pollution settled in over London last week, experts warned those with health problems to avoid strenuous exercise. The advice to Londoners essentially boiled down to this: breathe less.

At the same time Paris mayor Anne Hidalgo joined officials from Madrid, Athens and Mexico City in announcing plans to get all diesel vehicles off the roads by 2025. Diesel is highly polluting, emitting far greater amounts of dangerous nitrogen dioxide and tiny pollution particles than petrol, and can cause cancer to heart attacks.